1. Field of the Invention
The present invention relates to a thermal barrier coating material, a gas turbine parts, and a gas turbine that have high durability and, more particularly, relates to the constitution of a ceramic layer used as a top coat for the thermal barrier coating material.
2. Description of the Related Art
It has been attempted in recent years to improve the thermal efficiency of thermal power generation as part of efforts to reduce energy consumption. To improve the thermal efficiency of a gas turbine used in thermal power generation, it is effective to raise the temperature at the gas inlet, which may often be set to about 1500° C. Operating a power generator at such a high temperature requires building stationary vanes and rotor blades that constitute the gas turbine and/or the wall of a combustor with materials that can endure the high temperature. The turbine blades are made of heat-resistant metal but cannot endure such high temperatures. Therefore, it is common in practice for heat-resistant metal to be coated, via a metal bonding layer, with a thermal barrier coating material (TBC) made by forming a ceramic layer made of an oxide ceramic by thermal spraying or other process, for the protection from high temperatures. For the ceramic layer, a ZrO2-based material, particularly YSZ (Yttria stabilized Zirconia), which is ZrO2 stabilized partially or completely with Y2O3 is preferably used for the relatively lower thermal conductivity and relatively higher thermal expansion coefficient thereof among various ceramic materials.
However, when the stationary vane and/or the rotor blade of a gas turbine are coated with a thermal barrier coating material formed from a ceramic layer made of YSZ, the inlet temperature of the gas turbine may exceed 1500° C. depending on the type of gas turbine, giving rise to the possibility of a part of the ceramic layer spalling off under harsh operating conditions, thus losing the heat resistance. While a gas turbine of closed construction that can collect carbon dioxide has recently been developed in order to reduce greenhouse gas emissions, it is expected that the inlet temperature will reach around 1700° C. and the surface temperature of the turbine blade will reach around 1300° C. in this type of gas turbine. Thus, it is required to further improve the heat resistance of thermal barrier coating material for turbine blades.